What I would do at this point
Bhang, you probably don't want to hear this, but this is what I would do at this point.
1) Barefeats tested the official Apple 5770 and 5870 for performance on the MP 1,1 and found there was no appreciable difference. Essentially, the MP 1,1 couldn't take advantage of the extra power of the 5870. The recommendation is to get a 5770 if you have a MP 1,1.
2) The 6870 is slightly slower than the 5870, but still relatively comparable, meaning that you are probably won't benefit from the extra speed of the 6870. And that's if it worked, which it apparently isn't.
3) The official Apple 5770 is about the same price, maybe ever so slightly higher, than the 6870.
So if they will perform basically the same, and cost basically the same, but the Apple 5770 will work 100% with the MP, it seems like you should return the 6870 and buy an official Apple 5770. If you are outside the return period, you should sell it used and buy an Apple 5770 used.
You will have essentially the same performance out of the 5770 since your MP 1,1 is a bottleneck anyway. You will have 100% compatibility without any tricks and no worries about future updates breaking the hack.
At this point, that's my recommendation to you. People are largely unable to help you because for whatever reason the 6870 just works for everyone else.
Alternatively, I would try more monitors if you can borrow any from your friends or family. If it turns out everybody's monitor works except for your Acer, I'd replace the Acer.